Many modern downhole logging tools use neutron generators. The neutron output of a typical generator may vary with time either through short term variations due to small instabilities in the neutron generation process, because of temperature variations or through longer term changes due to the aging of the generator, in general, and its vacuum tube, in particular.
Some downhole applications use radioisotopic neutron sources such as 241AmBe or 252Cf. For these sources, the changes of the neutron output are determined principally by the half-life of the parent radioisotope. Therefore, the change is predictable given that the decay or decay chain of the isotopes are known. It is therefore possible to use an initial calibration of the source strength and to predict the neutron output at a later time from it. Often, calibrations are repeated at regular intervals to keep track of the changes in neutron output, in particular for sources with a short half-life like 252Cf, which has a 2.6-year half-life.
For electronic neutron generators, be it CW generators of neutrons or pulsed neutron generators, the neutron output is less predictable due to the nature of the neutron generation process. Typically, neutrons are generated in a nuclear fusion reaction in a sealed vacuum tube, which is coupled to one or more high voltage sources. Ions are accelerated on a target in the vacuum tube and the energetic ions may fuse with nuclei of the target material, and this may lead to the generation of neutrons.
The principal interaction used in downhole logging is the fusion of a deuteron (deuterium nucleus) with a triton (tritium nucleus), which creates an alpha particle (4He nucleus) and an energetic (14.1 MeV) neutron. In a typical generator tube, deuterium nuclei (d) or ionized deuterium molecules (D2+) are accelerated by high voltage potential differences of thirty kV to several hundred kV. The high acceleration voltage is required to impart the deuterium nucleus enough energy to overcome the Coulomb repulsion by the nucleus it is reacting with. The neutron output of a generator tube varies strongly with applied high voltage, internal gas pressure, target temperature and age to name a few. The neutron output may therefore change rapidly due to short term changes in the generator tube or the electronics controlling it. Such a change may be caused by parasitic electron emission in the tube due to the high electric fields or the interaction of the particle beam (d, T for example) with materials in the tube resulting in electron emission. Some events may lead to internal or external arcing and the neutron emission may cease almost entirely for short periods of time.
The neutron output of a neutron generator may be measured using a fast neutron detector in close proximity to the neutron source. Such a detector may be a plastic scintillation detector as described in U.S. Pat. No. 6,884,994, a solid state detector as described in U.S. patent application 2009/0057545, gas counter as indicated in U.S. patent publication 2011/0260044, all of which are assigned to the assignee of this disclosure, or another device that generates an output signal that is accurately related to the neutron output.
All of the above mentioned detectors rely on the detection of fast neutrons. Depending on the size and detection efficiency of the neutron monitor and the distance of the monitor from the source, the counting statistics in the neutron monitor may be low. In the absence of background subtraction, the uncertainty of a total number of counts is the square root of the number of counts. If a detector registers but 100 counts over a predetermined time period, the statistical uncertainty of the number of counts is ±10 counts (1 sigma). If precision of ±1% is required, 10,000 counts are desired and therefore an acquisition time larger by a factor of 100 is used.
In many cases, there is a desire to have a precise determination of the neutron output during a shorter time period and there is a desire for improving the precision over a shorter time period with a minimal impact on the accuracy of the neutron output measurement. Consequently, new developments in the field of output monitoring of radiation generators are desired.